Emberland et al. BMC Public Health (2017) 17:56 DOI 10.1186/s12889-016-3921-0
RESEARCH ARTICLE
Open Access
Psychological, social, and mechanical work exposures and disability retirement: a prospective registry study Jan S. Emberland*, Morten Birkeland Nielsen and Stein Knardahl
Abstract Background: Relations between several occupational psychological and social factors and disability retirement remain largely unexplored. Knowledge of which specific aspects of the work environment that affect risk of disability is a prerequisite for the success of organizational interventions aiming to prevent premature work force exit. The objective of the present study was to determine contributions to registered disability retirement by a broad range of psychological and social work exposures while taking into account effects of mechanical exposure. Methods: Written consent was obtained from 13 012 employees (96 organizations) representing a wide range of occupations, to link their survey responses to data from the Norwegian national registry of disability compensation. Median follow-up time was 5.8 years. To determine effects of self-reported work exposures on risk of disability retirement hazard ratios (HR) and 99% confidence intervals (99% CI) were calculated with Cox regression analysis. Effects of sex, age group, skill level, sickness absence in the last three years, and work exposures estimated to be confounders were accounted for. Post hoc stratification by sex was conducted to explore if identified predictors affected risk of disability retirement differently in men compared to women. Results: Contributors to higher risk of disability retirement were “role conflict” (high level HR 1.55 99% CI 1.07 to 2.24) and “physical workload” (high level HR 1.93 99% CI 1.39 to 2.68). Contributors to lower risk of disability retirement were “positive challenge” (high level HR 0.56 99% CI 0.34 to 0.93), “fair leadership” (high level HR 0.56 99% CI 0.39 to 0.81), and “control over work intensity” (high level HR 0.62, 99% CI 0.47 to 0.82). Direction of effects was not dependent on sex in any of the five identified predictors. Conclusions: Several specific psychological and social work factors are independent contributors to risk of disability retirement. In order to prevent premature work force exit workplace interventions should consider targeting the predictors identified by the present study. Keywords: Disability retirement, Occupational, Psychosocial, Mechanical, Registry data, Prospective, Hazard ratio
Background Exit from working life due to disability incurs large costs for societies as well as challenges to the quality of life of persons. Premature exit may result from impacts of biological/medical, psychological, and social conditions on functioning [1]. Still, the potential influences of a variety of non-physical work environment conditions on disability retirement have previously been devoted little attention. The present study aimed to determine which of a broad * Correspondence: [email protected] Department of Work Psychology and -Physiology, National Institute of Occupational Health, P.O. Box 8149 DepN-0033 Oslo, Norway
range of specific occupational psychological and social factors that may contribute to disability retirement. The workplace is an arena where individuals face challenges inherent in work tasks and social interactions. Work also provides opportunities for positive experiences from achievement and friendship. For many people, the job is a major source of feedback, fulfillment, and personal identity which in turn may promote work motivation, health, and work ability. Hence, conditions at work may contribute to disability retirement in several ways, e.g. by (I) influencing health and work ability, by (II) influencing motivation to
© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
Emberland et al. BMC Public Health (2017) 17:56
work, or by (III) making work too demanding relative to work ability (competence) in the jobs that are available. A 2011 systematic review of psychological and social work factors contributing to disability retirement [2] revealed that only a limited number of factors have been examined. The majority of the reviewed studies sought to identify contributions of “low control,” “psychological demands,” and “social support.” In many of these, as well as subsequent studies on registered work disability (e.g. [3–5]) it is apparent that the Job Content Questionnaire (JCQ; [6]) has been the preferred instrument to measure psychological and social work factors. This instrument groups several specific factors under broad demand and control dimensions, which raises the possibility that factors with opposite effects are grouped together under the same heading. However, knowledge of specific contributors to disability is arguably a prerequisite for developing practical interventions at the workplace. Recent studies have shown that psychological and social factors other than the dimensions assessed with the JCQ instrument contribute to health symptoms (e.g. [7, 8]) and perceived work ability [9]. Hence, it seems timely to broaden the scope by exploring contributions to disability retirement by a wider range of non-physical work factors. Biomechanical workload has received considerable attention in relation to registered work disability, and several studies have examined effects of both mechanical and psychological/social work factors [4, 5, 10, 11]. Based on previous publications it remains somewhat unclear if mechanical workload plays an important role in explaining associations between non-physical occupational factors and work disability. While the main objective was to determine contributions of a broad range of psychological and social work factors to publicly registered disability retirement, the present study also sought to account for effects of mechanical work exposure.
Page 2 of 11
the Norwegian Data Inspectorate. The organizations from which employees were recruited provided data on employees’ departmental affiliation, home address, and occupational title according to the Norwegian standard classification of the occupations (STYRK) - a system developed by Statistics Norway based on the International Classification of Occupation (ISCO-88). In return for participation in the project, the organizations received written reports and oral presentations of results with the objective of supporting management and personnel in the process of monitoring their work conditions. Population
Employees were recruited from 96 companies representing a broad spectrum of occupational sectors including health care, education, government and public administration, engineering, project management, industry, and non-profit organizations. A total of 30 585 subjects were invited to participate in the period 2004 to 2014. At the time of invitation 28 833 (94.3%) subjects were aged 18– 62 years and thus eligible for disability retirement benefits. Subjects above the age of 62 were excluded as they are additionally entitled to early age pension. Of the invited aged 18-62, 17 789 (61.7%) responded to the questionnaire survey. Written consent was obtained from 13 012 (73.1%) subjects and enabled linking these individuals’ questionnaire responses to data from the national registry of disability compensation maintained by the Norwegian Labour and Welfare Administration (NAV). Based on information from this registry subjects were excluded from the present study if having incomplete history of work disability (emigration before response, n = 76; 0.6%) or having received disability retirement compensation (due to some proportion of disability) prior to response (n = 498; 3.8%). Thus, the final cohort (Fig. 1) consisted of 12 438 subjects (95.6% of the participants; mean age 41.8 years; 55.5% women) with a median follow-up time of 5.8 years.
Methods Study design
Ethical approval
The current study is part of a full-panel prospective study of work factors contributing to health, work ability, absence, and exit from working life in Norway. Employees were invited to participate in a web-based survey containing questions on background information, psychological, social, and mechanical work factors, work organization, mastery of work, attitudes towards work, organizational change, personality, health behavior, coping strategies, mental health, health complaints, and work ability. Each employee received a letter containing information about the survey and a personalized code for logging into the web-based questionnaire. A paper version of the questionnaire was made available upon request. Written information specified the strict confidentiality guidelines and informed about the license for data collection granted by
The project has been approved by the Regional Committees for Medical and Health Research Ethics (REC) in Norway and has permission from The Norwegian Data Protection Authority. The research was carried out in compliance with the World Medical Association Declaration of Helsinki and written informed consent was obtained from all study participants (for details see [12]). Outcome: disability retirement
Disability retirement was defined as the subject receiving disability pension compensation from the Norwegian Labour and Welfare Administration (NAV). Unlike reimbursement for sickness absence, the requirement for receiving disability pension is that the magnitude of work inability is at least 50% [13]. Compensation criteria
Emberland et al. BMC Public Health (2017) 17:56
Page 3 of 11
The psychological and social scales covered job demands, control at work, predictability at work, role expectation, leadership, and organizational culture and climate (see Table 3). A complete list of scale items has been published elsewhere [14, 15]. Responses to items were given on a 5-point scale; 1 “very seldom or never,” 2 “somewhat seldom,” 3 “sometimes,” 4 “somewhat often,” and 5 “very often or always.” Mechanical exposure was measured with two items; physical workload, and working with arms raised to or above shoulder level (single item). Physical workload was assessed by measuring the extent to which subjects were lifting or handling objects that weigh approximately 1–5 kg, 6–15 kg, and more than approximately 15 kg with own muscular strength. Response categories for both measures were; 1 “seldom or never,” 2 “sometimes,” 3 “daily,” and 4 “many times per day.” To address dose-response relationships, scale scores were categorized into three exposure levels. In line with previously published analyses from parts of the same cohort [8, 9] scores from 1.00 through 2.60 were classified as “low”; 2.61 through 3.40 as “middle”; and 3.41 through 5.00 were classified as “high” exposure, respectively. For the four-level measures (mechanical exposures) scores from 1.00 through 2.50 were classified as “low”; 2.51 through 3.50 as “middle,” and 3.51 through 4.00 as “high” exposure, respectively. Statistical analysis
Fig. 1 Flow diagram depicting the selection process. aEmployees above 62 years of age are additionally eligible for statutory early age pension and thus excluded from the present study. bNational registers maintained by the Norwegian Labour and Welfare Administration
must be substantiated by an extensive physician’s certificate and confirmed by examinations (which includes assessment of job specific capacities) undertaken by specialized (usually medical) representatives from a local NAV office. For the respondents consenting to registry linkage we had access to information on disability pension compensation recorded in the NAV registry up to 1 January 2015 (the end of the present study’s follow-up period). Psychological, social, and mechanical work factors
Psychological and social exposures were measured by the General Nordic Questionnaire for psychological and social factors at work (QPSNordic). Psychometric evaluations of QPSNordic have shown high validity and reliability of the scales included in the present study [14]. Reliability analyses indicate that the factors are consistent across a wide range of occupational groups [15].
Statistical analyses were conducted with SPSS version 22.0 (IBM, Armonk, NY) and with the survival package [16] for R version 3.2.2. Non-response analysis was conducted to determine if background variables influenced whether subjects responded at baseline. Binary logistic regressions were run with response status as outcome. The effects of age, sex, occupational group, and skill level were estimated separately. Since the occupational group variable does not have a clear intrinsic order this variable was treated as nominal. Thus, the effects of each of the 10 occupational categories on response status were calculated by using the combination of all other occupational groups as reference. Hazard ratios were calculated with Cox regression analysis to determine effects of background variables and 16 work exposures on disability retirement. Analyses were run separately with each background/work exposure variable as independents. Since multiple analyses were performed 99% confidence intervals were chosen in order to reduce the risk of type 1 error. Due to the clearly higher hazard ratio for subsequent work disability in women (Table 2) post hoc analysis with stratification by sex was conducted to uncover if the identified predictors in the primary analysis (see Table 3) were differentially related to
Emberland et al. BMC Public Health (2017) 17:56
work disability according to sex. In order to calculate a single effect estimate for each predictor (to be compared across sexes), exposure variables were entered as continuous in these analyses. As recommended for studies of healthy populations [17] attained age (at censoring/event) was the underlying time scale in the analyses (except in the non-response analysis with age group as the primary predictor) rather than “time-on-study” (i.e. years since baseline response). Interactions between each background/work exposure variable and the logarithm of the follow-up time i.e. age in years, (for age group; time since response) were determined by tests of non-zero slopes followed by graphical plots of scaled Schoenfeld residuals. The tests indicated no violations of the proportional hazards assumption (for all variables: P >0.05). Follow-up time was calculated from the calendar date subjects responded to the web-based questionnaire. For subjects completing the paper version of the questionnaire (n = 1106) and for some of the web responders (n = 882) information on actual response date was not obtainable. In these cases response date was set to the last possible date of response for all employees in their respective company. Follow-up ended at the time point of being granted disability pension (n = 553), emigration (n = 42), death (n = 39), or 1 January 2015 (n = 10 407) whichever came first. At the time of this study data from the old age pension registry had not been obtained. Thus, subjects were also censored if they reached the eligible age for early statutory pension (which is the first day of the month following the 62nd birthday) before the end of follow-up (n = 1397). Potential confounding variables
The effects of sex, skill level, age group, and sickness absence (in the 3 years preceding baseline response) were accounted for in all analyses of work exposures and disability retirement. Skill level was determined by recoding the occupational groups (ISCO-88) in agreement with the International Standard for Classification of Education (ISCED). In cases where no information on occupational group (ISCO-88) had been provided by the subjects’ respective companies, missing values were substituted with self-reported skill level information (n = 102). Baseline age was categorized into; 18–29, 30–39, 40–49, and 50– 62 years, to account for potential cohort effects [17]. Some chronic illnesses may lead to reductions in certain work exposure levels [18, 19]. Also, a number of chronic diseases are linked with risk of disability pensioning [20]. Thus, the analyses were adjusted for sickness absence prior to baseline response caused by cardiovascular disease, respiratory illnesses, cerebrovascular disease, diabetes, or cancer [21, 22]. This information was obtained from the sickness absence registry maintained by the Norwegian
Page 4 of 11
Labour and Welfare Administration and limited to the 3 years prior to baseline participation. The observed effect of each work exposure on work disability may be confounded by the effects of other work exposures. However, since the scales are interrelated [14, 15] simultaneously controlling for all other work exposures is likely to result in overadjustment. Thus, a strategy described by Rothman et al. [23] was utilized to identify influence by other work exposures above a certain threshold level. First, the effect of an exposure on disability retirement was estimated. Subsequently, one other exposure was added to the model. If the effect estimate of the original exposure changed by more than 10% in the second model the added exposure was defined as a confounder [23]. This procedure was carried out for each of the 16 work exposure measures. The influence of each of the other 15 work exposure measures was estimated.
Results Non-response analysis
Sex was not predictive of responding at baseline (P >0.05; Table 1). Subjects aged >29 (i.e. 30–39, 40–49, and 50–62) exhibited higher odds of responding compared to the lowest age group (for all: P
RESEARCH ARTICLE
Open Access
Psychological, social, and mechanical work exposures and disability retirement: a prospective registry study Jan S. Emberland*, Morten Birkeland Nielsen and Stein Knardahl
Abstract Background: Relations between several occupational psychological and social factors and disability retirement remain largely unexplored. Knowledge of which specific aspects of the work environment that affect risk of disability is a prerequisite for the success of organizational interventions aiming to prevent premature work force exit. The objective of the present study was to determine contributions to registered disability retirement by a broad range of psychological and social work exposures while taking into account effects of mechanical exposure. Methods: Written consent was obtained from 13 012 employees (96 organizations) representing a wide range of occupations, to link their survey responses to data from the Norwegian national registry of disability compensation. Median follow-up time was 5.8 years. To determine effects of self-reported work exposures on risk of disability retirement hazard ratios (HR) and 99% confidence intervals (99% CI) were calculated with Cox regression analysis. Effects of sex, age group, skill level, sickness absence in the last three years, and work exposures estimated to be confounders were accounted for. Post hoc stratification by sex was conducted to explore if identified predictors affected risk of disability retirement differently in men compared to women. Results: Contributors to higher risk of disability retirement were “role conflict” (high level HR 1.55 99% CI 1.07 to 2.24) and “physical workload” (high level HR 1.93 99% CI 1.39 to 2.68). Contributors to lower risk of disability retirement were “positive challenge” (high level HR 0.56 99% CI 0.34 to 0.93), “fair leadership” (high level HR 0.56 99% CI 0.39 to 0.81), and “control over work intensity” (high level HR 0.62, 99% CI 0.47 to 0.82). Direction of effects was not dependent on sex in any of the five identified predictors. Conclusions: Several specific psychological and social work factors are independent contributors to risk of disability retirement. In order to prevent premature work force exit workplace interventions should consider targeting the predictors identified by the present study. Keywords: Disability retirement, Occupational, Psychosocial, Mechanical, Registry data, Prospective, Hazard ratio
Background Exit from working life due to disability incurs large costs for societies as well as challenges to the quality of life of persons. Premature exit may result from impacts of biological/medical, psychological, and social conditions on functioning [1]. Still, the potential influences of a variety of non-physical work environment conditions on disability retirement have previously been devoted little attention. The present study aimed to determine which of a broad * Correspondence: [email protected] Department of Work Psychology and -Physiology, National Institute of Occupational Health, P.O. Box 8149 DepN-0033 Oslo, Norway
range of specific occupational psychological and social factors that may contribute to disability retirement. The workplace is an arena where individuals face challenges inherent in work tasks and social interactions. Work also provides opportunities for positive experiences from achievement and friendship. For many people, the job is a major source of feedback, fulfillment, and personal identity which in turn may promote work motivation, health, and work ability. Hence, conditions at work may contribute to disability retirement in several ways, e.g. by (I) influencing health and work ability, by (II) influencing motivation to
© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
Emberland et al. BMC Public Health (2017) 17:56
work, or by (III) making work too demanding relative to work ability (competence) in the jobs that are available. A 2011 systematic review of psychological and social work factors contributing to disability retirement [2] revealed that only a limited number of factors have been examined. The majority of the reviewed studies sought to identify contributions of “low control,” “psychological demands,” and “social support.” In many of these, as well as subsequent studies on registered work disability (e.g. [3–5]) it is apparent that the Job Content Questionnaire (JCQ; [6]) has been the preferred instrument to measure psychological and social work factors. This instrument groups several specific factors under broad demand and control dimensions, which raises the possibility that factors with opposite effects are grouped together under the same heading. However, knowledge of specific contributors to disability is arguably a prerequisite for developing practical interventions at the workplace. Recent studies have shown that psychological and social factors other than the dimensions assessed with the JCQ instrument contribute to health symptoms (e.g. [7, 8]) and perceived work ability [9]. Hence, it seems timely to broaden the scope by exploring contributions to disability retirement by a wider range of non-physical work factors. Biomechanical workload has received considerable attention in relation to registered work disability, and several studies have examined effects of both mechanical and psychological/social work factors [4, 5, 10, 11]. Based on previous publications it remains somewhat unclear if mechanical workload plays an important role in explaining associations between non-physical occupational factors and work disability. While the main objective was to determine contributions of a broad range of psychological and social work factors to publicly registered disability retirement, the present study also sought to account for effects of mechanical work exposure.
Page 2 of 11
the Norwegian Data Inspectorate. The organizations from which employees were recruited provided data on employees’ departmental affiliation, home address, and occupational title according to the Norwegian standard classification of the occupations (STYRK) - a system developed by Statistics Norway based on the International Classification of Occupation (ISCO-88). In return for participation in the project, the organizations received written reports and oral presentations of results with the objective of supporting management and personnel in the process of monitoring their work conditions. Population
Employees were recruited from 96 companies representing a broad spectrum of occupational sectors including health care, education, government and public administration, engineering, project management, industry, and non-profit organizations. A total of 30 585 subjects were invited to participate in the period 2004 to 2014. At the time of invitation 28 833 (94.3%) subjects were aged 18– 62 years and thus eligible for disability retirement benefits. Subjects above the age of 62 were excluded as they are additionally entitled to early age pension. Of the invited aged 18-62, 17 789 (61.7%) responded to the questionnaire survey. Written consent was obtained from 13 012 (73.1%) subjects and enabled linking these individuals’ questionnaire responses to data from the national registry of disability compensation maintained by the Norwegian Labour and Welfare Administration (NAV). Based on information from this registry subjects were excluded from the present study if having incomplete history of work disability (emigration before response, n = 76; 0.6%) or having received disability retirement compensation (due to some proportion of disability) prior to response (n = 498; 3.8%). Thus, the final cohort (Fig. 1) consisted of 12 438 subjects (95.6% of the participants; mean age 41.8 years; 55.5% women) with a median follow-up time of 5.8 years.
Methods Study design
Ethical approval
The current study is part of a full-panel prospective study of work factors contributing to health, work ability, absence, and exit from working life in Norway. Employees were invited to participate in a web-based survey containing questions on background information, psychological, social, and mechanical work factors, work organization, mastery of work, attitudes towards work, organizational change, personality, health behavior, coping strategies, mental health, health complaints, and work ability. Each employee received a letter containing information about the survey and a personalized code for logging into the web-based questionnaire. A paper version of the questionnaire was made available upon request. Written information specified the strict confidentiality guidelines and informed about the license for data collection granted by
The project has been approved by the Regional Committees for Medical and Health Research Ethics (REC) in Norway and has permission from The Norwegian Data Protection Authority. The research was carried out in compliance with the World Medical Association Declaration of Helsinki and written informed consent was obtained from all study participants (for details see [12]). Outcome: disability retirement
Disability retirement was defined as the subject receiving disability pension compensation from the Norwegian Labour and Welfare Administration (NAV). Unlike reimbursement for sickness absence, the requirement for receiving disability pension is that the magnitude of work inability is at least 50% [13]. Compensation criteria
Emberland et al. BMC Public Health (2017) 17:56
Page 3 of 11
The psychological and social scales covered job demands, control at work, predictability at work, role expectation, leadership, and organizational culture and climate (see Table 3). A complete list of scale items has been published elsewhere [14, 15]. Responses to items were given on a 5-point scale; 1 “very seldom or never,” 2 “somewhat seldom,” 3 “sometimes,” 4 “somewhat often,” and 5 “very often or always.” Mechanical exposure was measured with two items; physical workload, and working with arms raised to or above shoulder level (single item). Physical workload was assessed by measuring the extent to which subjects were lifting or handling objects that weigh approximately 1–5 kg, 6–15 kg, and more than approximately 15 kg with own muscular strength. Response categories for both measures were; 1 “seldom or never,” 2 “sometimes,” 3 “daily,” and 4 “many times per day.” To address dose-response relationships, scale scores were categorized into three exposure levels. In line with previously published analyses from parts of the same cohort [8, 9] scores from 1.00 through 2.60 were classified as “low”; 2.61 through 3.40 as “middle”; and 3.41 through 5.00 were classified as “high” exposure, respectively. For the four-level measures (mechanical exposures) scores from 1.00 through 2.50 were classified as “low”; 2.51 through 3.50 as “middle,” and 3.51 through 4.00 as “high” exposure, respectively. Statistical analysis
Fig. 1 Flow diagram depicting the selection process. aEmployees above 62 years of age are additionally eligible for statutory early age pension and thus excluded from the present study. bNational registers maintained by the Norwegian Labour and Welfare Administration
must be substantiated by an extensive physician’s certificate and confirmed by examinations (which includes assessment of job specific capacities) undertaken by specialized (usually medical) representatives from a local NAV office. For the respondents consenting to registry linkage we had access to information on disability pension compensation recorded in the NAV registry up to 1 January 2015 (the end of the present study’s follow-up period). Psychological, social, and mechanical work factors
Psychological and social exposures were measured by the General Nordic Questionnaire for psychological and social factors at work (QPSNordic). Psychometric evaluations of QPSNordic have shown high validity and reliability of the scales included in the present study [14]. Reliability analyses indicate that the factors are consistent across a wide range of occupational groups [15].
Statistical analyses were conducted with SPSS version 22.0 (IBM, Armonk, NY) and with the survival package [16] for R version 3.2.2. Non-response analysis was conducted to determine if background variables influenced whether subjects responded at baseline. Binary logistic regressions were run with response status as outcome. The effects of age, sex, occupational group, and skill level were estimated separately. Since the occupational group variable does not have a clear intrinsic order this variable was treated as nominal. Thus, the effects of each of the 10 occupational categories on response status were calculated by using the combination of all other occupational groups as reference. Hazard ratios were calculated with Cox regression analysis to determine effects of background variables and 16 work exposures on disability retirement. Analyses were run separately with each background/work exposure variable as independents. Since multiple analyses were performed 99% confidence intervals were chosen in order to reduce the risk of type 1 error. Due to the clearly higher hazard ratio for subsequent work disability in women (Table 2) post hoc analysis with stratification by sex was conducted to uncover if the identified predictors in the primary analysis (see Table 3) were differentially related to
Emberland et al. BMC Public Health (2017) 17:56
work disability according to sex. In order to calculate a single effect estimate for each predictor (to be compared across sexes), exposure variables were entered as continuous in these analyses. As recommended for studies of healthy populations [17] attained age (at censoring/event) was the underlying time scale in the analyses (except in the non-response analysis with age group as the primary predictor) rather than “time-on-study” (i.e. years since baseline response). Interactions between each background/work exposure variable and the logarithm of the follow-up time i.e. age in years, (for age group; time since response) were determined by tests of non-zero slopes followed by graphical plots of scaled Schoenfeld residuals. The tests indicated no violations of the proportional hazards assumption (for all variables: P >0.05). Follow-up time was calculated from the calendar date subjects responded to the web-based questionnaire. For subjects completing the paper version of the questionnaire (n = 1106) and for some of the web responders (n = 882) information on actual response date was not obtainable. In these cases response date was set to the last possible date of response for all employees in their respective company. Follow-up ended at the time point of being granted disability pension (n = 553), emigration (n = 42), death (n = 39), or 1 January 2015 (n = 10 407) whichever came first. At the time of this study data from the old age pension registry had not been obtained. Thus, subjects were also censored if they reached the eligible age for early statutory pension (which is the first day of the month following the 62nd birthday) before the end of follow-up (n = 1397). Potential confounding variables
The effects of sex, skill level, age group, and sickness absence (in the 3 years preceding baseline response) were accounted for in all analyses of work exposures and disability retirement. Skill level was determined by recoding the occupational groups (ISCO-88) in agreement with the International Standard for Classification of Education (ISCED). In cases where no information on occupational group (ISCO-88) had been provided by the subjects’ respective companies, missing values were substituted with self-reported skill level information (n = 102). Baseline age was categorized into; 18–29, 30–39, 40–49, and 50– 62 years, to account for potential cohort effects [17]. Some chronic illnesses may lead to reductions in certain work exposure levels [18, 19]. Also, a number of chronic diseases are linked with risk of disability pensioning [20]. Thus, the analyses were adjusted for sickness absence prior to baseline response caused by cardiovascular disease, respiratory illnesses, cerebrovascular disease, diabetes, or cancer [21, 22]. This information was obtained from the sickness absence registry maintained by the Norwegian
Page 4 of 11
Labour and Welfare Administration and limited to the 3 years prior to baseline participation. The observed effect of each work exposure on work disability may be confounded by the effects of other work exposures. However, since the scales are interrelated [14, 15] simultaneously controlling for all other work exposures is likely to result in overadjustment. Thus, a strategy described by Rothman et al. [23] was utilized to identify influence by other work exposures above a certain threshold level. First, the effect of an exposure on disability retirement was estimated. Subsequently, one other exposure was added to the model. If the effect estimate of the original exposure changed by more than 10% in the second model the added exposure was defined as a confounder [23]. This procedure was carried out for each of the 16 work exposure measures. The influence of each of the other 15 work exposure measures was estimated.
Results Non-response analysis
Sex was not predictive of responding at baseline (P >0.05; Table 1). Subjects aged >29 (i.e. 30–39, 40–49, and 50–62) exhibited higher odds of responding compared to the lowest age group (for all: P
